The present invention relates to an electrostatic relay incorporating an electrostatic actuator which uses electrostatic attractive force as a drive source thereof.
An electrostatic relay is different from a conventional electromagnetic relay which uses an electromagnet in a structure that electrostatic attractive force is used as drive force to open/close contacts thereof. Since any coil for generating electromagnetic force is not required, the number of mechanical elements can be reduced. Moreover, the size reduction can be permitted. Since the electrostatic actuator, which is intrinsically a capacitor, is employed as the drive source, electric power consumption can be reduced. Therefore, research and development have been performed to put the electrostatic relay to practical use.
An electrostatic relay of the foregoing type has been disclosed in, for example, Japanese Patent Laid-Open No. 2-100224. That is, single crystal Si is selectively etched so that an elastic torsion bar and a seesaw structure connected to the elastic torsion bar are formed. Moreover, a movable electrode of the electrostatic actuator and a movable contact of the relay are provided for the seesaw structure so as to be disposed on an electrically insulating substrate through a spacer, the electrically insulating substrate having a fixed electrode and a fixed contact disposed at opposite positions.
The above-mentioned electrostatic relay has a structure that the elastic torsion bar is twisted when voltage is applied between the fixed electrode and the movable electrode when the electrostatic relay is operated. Thus, the seesaw structure in the portion applied with the voltage performs a rotational motion so that the movable contact is brought into contact with the fixed contact.
The above-mentioned conventional electrostatic relay has the following problem when a relay contact in the form of a pair of the movable contact and a fixed contact is opened/closed.
When the relay contact is closed, the conventional electrostatic relay is arranged such that voltage is applied between the movable electrode of the electrostatic actuator provided for the seesaw structure adjacent to the target contact and the fixed electrode adjacent to the substrate. Electrostatic attractive force acting between the two electrodes is used to cause the seesaw structure to perform a seesaw motion such that the elastic torsion bar serves as a fulcrum of rotation. Thus, the movable contact is brought into contact with the fixed contact so that the contact is closed.
Since the foregoing electrostatic relay has the movable contact provided for the end of the seesaw structure, the movable contact is brought into contact with the fixed contact in a state in which the movable contact is inclined. Therefore, a satisfactorily large contact area cannot be realized and the contact resistance is raised excessively. If the position of the structure which is provided with the movable contact is not appropriately selected, the structure is undesirably brought into contact together with the movable contact. Thus, there is a possibility that the pressure between the contacts becomes insufficiently low.
The electrostatic relay having the above-mentioned structure is formed such that the movable electrode opposite to the fixed electrode is formed adjacent to the fulcrum of rotation as compared with the movable contact of the seesaw structure held by the elastic torsion bar such that a gap is maintained. Therefore, when the movable contact has been brought into contact with the fixed contact and thus the rotational motion of the seesaw structure has been interrupted, a wedge shape air gap is generated between the fixed electrode and the movable electrode.
However, the electrostatic attractive force is in proportion to the inverse square of the gap between the two electrodes. Therefore, the electrostatic actuator encounters reduction in the electrostatic attractive force owning to the great air gap even during the suction operation. Since a sufficiently high pressure is not applied to the contact, the resistance of the contact cannot satisfactorily be reduced. If the operating voltage is raised to overcome the foregoing problem, the practicality of the electrostatic relay excessively deteriorates.
If the resistance of the contact is high, the contact is overheated by dint of Joule heat when a contact electric current is supplied. Thus, a phenomenon that the contact is melted easily takes place. When the operating voltage is raised to reduce the high contact resistance so as to raise the contact pressure, the practicality of the electrostatic relay excessively deteriorates.
When the relay contact is opened, the following problem arises.
That is, when the relay contact is opened, the movable contact and the fixed contact must be separated from each other. In the foregoing case, the fixed electrode and the movable electrode of the electrostatic actuator are short-circuited to make the electrostatic attractive force between the electrodes to be zero. As a result, restoring force of the elastic torsion bar which rotatably supports the seesaw structure acts so that the movable contact is moved upwards. Thus, the contact with the fixed contact is suspended.
As described above, when the relay contact of the conventional electrostatic relay is opened, only the restoring force of the elastic torsion bar serving as the torsional elastic member is the separating force. If a high contact electric current is applied and thus the contact is melted, the force for forcibly separating the contacts from each other is insufficiently small.
To prevent the above-mentioned fact, the restoring force of the elastic torsion bar is required to be enlarged. In the foregoing case, also the force for closing the relay contact is enlarged. Therefore, the voltage which must be applied to the electrostatic relay must be raised. Thus, the practicality of the electrostatic relay excessively deteriorates.
As a method of enlarging the force for opening the relay contact, a method may be employed whereby voltage is applied between the fixed electrode and the movable electrode of the electrostatic actuator (hereinafter called an "opposite electrode") opposite to the closed contact of the seesaw structure of the electrostatic relay so as to generate electrostatic attractive force. Thus, the force for upward moving the seesaw structure closing the contact is generated.
However, since the movable electrode of the electrostatic actuator of the opposite electrode is moved upwards, the distance from the fixed electrode is elongated.
Since the force for rotating the seesaw structure is the force of a lever, the force is a product of a distance from the central axis of rotation and the attractive force at the foregoing position. The distance between the fixed electrode and the movable electrode is elongated in proportion to the distance from the central axis of rotation. Since the electrostatic attractive force acting at the electrodes is in proportion to the inverse square of the distance between the electrodes, the attractive force of the electrostatic actuator at the opposite electrode is reduced excessively. Therefore, the foregoing attractive force cannot satisfactorily separate the contacts from each other. Thus, if the voltage which is applied to the opposite electrode is not raised, the force for separating the relay contact cannot easily be enlarged.
As described above, the conventional electrostatic relay encounters excessively high contact resistance when the contact has been closed. Thus, the phenomenon that the contact is melted easily takes place. Since the force for separating the contacts from each other is too small, a failure that the contacts are melted takes place if the contacts are melted. Therefore, a satisfactorily high contact electric current cannot be maintained. Thus, the reliability and practicality are unsatisfactory. To solve the above-mentioned problems, the voltage for operating the electrostatic relay must be raised. However, the high operating voltage excessively deteriorates the practicality of the electrostatic relay.